1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to systems and methods for increasing extraction efficiency in lasers.
2. Description of the Related Art
Power efficiency is a critical issue for many, weapon-class solid-state high energy laser (HEL) systems, specifically for those integrated into mobile platforms. Power efficiency determines, in the long run, system applicability to mobile platform and battlefield conditions where power sources and waste energy management subsystem resources are limited. Diode-pumped solid-state laser systems are preferred for many applications due to the characteristic efficiency thereof. Proper choice of active medium and doping concentration, of pumping diodes and pumping geometry, of heat sink monitoring, etc., have facilitated high efficiencies for solid-state lasers. These are methods for minimizing loss at the stage of transforming the external source power into power stored as optical excitation of electrons in the solid state matrix.
Another source of efficiency loss comes from the next step: the extraction of laser medium excitation power into the power of the amplified output signal. Efficiency of the laser amplifier stored power extraction is often deteriorated by a non-uniformity of the intensity of the amplified laser beam. In practice, a laser beam intensity pattern is usually non-uniform or fine structured inside the active elements of HEL systems because of the coexistence of two factors: optical aberrations and the spatial coherence of laser radiation. Fine structures of different types, such as coherent beam caustics, speckles, and interference fringes, reduce the bulk filling factor or the overlap of the laser beam with the amplifying medium due to multiple local spots or areas where the laser light does not saturate the medium and does not extract the stored energy. Power stored at those spots/areas does not contribute to signal amplification but is wasted by fluorescence and amplified spontaneous emission (ASE). The total results of such negative effects can exceed 35% in reduction of the maximal possible extraction efficiency.
Prior attempts to increase the extraction efficiency of optical amplifiers were mostly targeted at eliminating the underfill effects caused by non-ideal geometrical overlaps between the laser beam and the laser active medium. These effects correspond to a large spatial scale comparable to the size of the active element. Solutions include: 1) matching the input beam footprint to the input entrance of the amplifier to exclude empty areas near the borders, 2) double-passing or multi-passing the same active volume and through neighboring paths cover the volume with high intensity signal, 3) integrated reflectors for amplifier slabs to arrange complementary paths at zigzagging.
Another effect, known as the hole burning effect, that reduces the amplifier extraction efficiency is due to the interference of counter propagating coherent laser beams and the subsequent creation of an intensity spatial modulation pattern of about a half-wavelength scale. A typical solution to minimize this effect is to operate the amplifier at multiple laser wavelengths or in short pulses without time-overlap inside the amplifier medium. No known attempts have been made to solve the problem of the underfill at the level of intensity occurring in fine structured laser beams due to coherent interference and diffraction propagation effects.
Hence, a need exists in the art for a system or method for reducing underfill effects due to fine structured laser beams to increase extraction efficiency in laser amplifiers.